...it seems self-evident that one ought to
instruct every God-fearing person to hasten to gather his
livestock and his horses at home [an allusion to Exodus 9:20]
and not to participate in horse-races — neither in establishing
them, nor by watching them: because of the pain to animals
caused thereby, because it is "a dwelling place of scoffers,"
and because it is "playing with dice" [that is, gambling].

Among the reasons the Chief Rabbi cited in drawing
his conclusion are the following:

Racing involves the death of many horses, and
this violates the Jewish law against wanton destruction.

Horse slaughter would create a risk that
horsemeat would be sold in Israel, which would violate Jewish
law.

Whoever shows compassion is shown compassion.

Using horses for racing is unnecessary; the
cruelty involved, only for the purpose of making some rich
people richer, is prohibited.

Judaism forbids gambling because it enriches
one person at the expense of another.